Deciphering Deep Ocean Circulation Changes Between the Present and the Last Glacial Maximum

The paleoclimate record indicates that the deep ocean circulation and water masses have undergone major rearrangements between glacial and interglacial climates, which have likely played an important role in the observed atmospheric carbon dioxide swings by affecting the partitioning of carbon between the atmosphere and ocean. The mechanisms by which the deep ocean circulation changed, however, are still unclear, which represents a major challenge to our understanding of past and future climates.

We address this question using a combination of theory and idealized numerical simulations. The results suggest that various inferred differences in the deep ocean circulation and stratification between glacial and interglacial climates can be attributed to increased Antarctic sea-ice formation in a colder world. Colder temperatures lead to thicker ice, which is exported by winds. The associated increased freshwater export leads to saltier and denser Antarctic Bottom Water, consistent with high abyssal salinities inferred for the Last Glacial Maximum (LGM). The enhanced deep ocean stratification moreover results in a shoaling of the inter-hemispheric overturning circulation, again consistent with proxy evidence for the LGM.

The results are also relevant for our understanding of the deep ocean circulation in a warmer world. Specifically, it is shown that the disappearance of Antarctic sea ice may lead to a shut-down of Antarctic bottom water formation, leaving the entire deep ocean filled with warm and very weakly stratified water of North Atlantic origin.